1. Field of the Invention
This invention concerns force measuring load cells, and more particularly, load cells adapted to resolve and measure all of the force and bending moment components capable of being exerted thereon.
2. Description of the Prior Art
Multi-axial load cells in which the components of multi-axial forces and moments may be resolved have commonly utilized crossing beam elements which have strain gages affixed thereto which are placed in bending by the various loads, which deflection is measured by the strain gages to measure the load applied thereto. In most of these load cells the crossing beams are fixed at both ends and a disadvantage of this arrangement is thereby created by the tension-compression loading of the beams not oriented so as to be in bending since the load-induced deflection is thus greatly reduced.
In a prior art load cell described in a Master's Thesis entitled "Force Feedback Steering of a Tele-Operator System," by R. C. Groome, Jr., MIT Draper Laboratory Report T-575, August 1972, an arrangement has been utilized in which the cantilever beams were allowed free sliding movement in directions aligned with their axes, so that the deflection range was increased since all elements were deflected only in bending. However, the device utilized strain gages to measure such bending and strain gages are relatively limited in the range of dynamic loads which they can measure.
In U.S. Pat. No. 3,939,704 issued Feb. 24, 1976 to Zipin and assigned to the same assignee as this application, a multi-axis load cell is disclosed which is capable of measuring a wide range of dynamic loads by virture of a cantilever beam arrangement similar to that disclosed in the MIT Report referred to above, combined with LVDT displacement transducers this combination allowing accurate measurement of a wide range of force components while being capable of resolving all components of the forces and bending moments applied thereto. However, the specific embodiment disclosed in that application suffers from the disadvantage that the various LVDT transducers are susceptible to being influenced by combinations of forces and moments applied to the load cell so that inaccuracies may result or signal corrections may be required. Furthermore, the particular cantilever bar ball joints connections were of a nature that hysteresis would be a problem.
Finally, this embodiment incorporated the use of eight separate LVDT transducers to resolve the various force and moment components, while only six such transducers are inherently necessary to resolve these components, this excess thereby increasing the cost of the load cell.
Therefore, it is an object of the present invention to produce a multi-axis load cell of a type described in which cross-influencing between the various transducers is minimized, in which the hysteresis is minimized, and in which the number of displacement transducers is reduced to a minimum.